Type 2 diabetes is characterized by hyperglycemia, insulin resistance, absolute or relative insulin deficiency, hyperglucagonemia, and increased hepatic glucose production. Although many treatment trials for type 2 diabetes have been held, there is still no definitive treatment for the disease. Insulin secretion is modulated by incretin hormones which are produced by the intestinal enteroendocrine cells and constitute one arm of the enteroinsular axis. There are two major incretins. One is glucose-dependent insulinotrophic polypepetide (GIP) and the other is glucagon like peptide-1 (GLP-1). These two incretin hormones account for 20% and 80% respectively, of the intestinal incretin effect. Holst J J: Glucagonlike peptide 1: a newly discovered gastrointestinal hormone. Gastroenterology 107:1848-1855, 1994 GIP, but not GLP-1, tends to lose its actions in patients with type 2 diabetes. Nauck M A, Heimesaat M M, Orskov C, Holst J J, Ebert R, Creutzfeldt W: Preserved incretin activity of glucagons-like peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus. J Clin Invest 91:301-307, 1993. GLP-1 was recently used for the treatment of type 2 diabetes. See U.S. Pat. Nos. 5,614,492; 5,545,618 and 6,048,724 which are incorporated herein by reference.
GLP-1, produced by intestinal L-cells, stimulates glucose induced insulin secretion and inhibits glucagon secretion. GLP-1 has two active forms, GLP-1 (7-36) amide and GLP-1 (7-37), that are products of posttranslational processing of proglucagon in mammalian intestinal cells. The active forms of GLP-1 are degradable in the plasma by the action of dipeptidyl peptidase IV. During degradation, the active form of GLP1(7-36 or 7-37) loses its N-terminal amino acid residues and results in an inactive form of GLP-1 (9-36 amide). Therefore, the active forms of GLP-1 have very short plasma half lives (about 5 minutes) and metabolic clearance rates. Fehmann H C et. al: Endocr Rev 16:390-410, 1995 There have been several studies on administration of GLP-1 to type 2 diabetic patients which have shown that GLP-1 effectively reduces hyperglycemia in type 2 diabetic patients. Nauck et. al. J Clin Invest 91:301-307, 1993; Nauck et. A1: Diabetes Care 21:1925-1931, 1998; and Rachman J et. al Diabetologia 40:205-211, 1997. However, it is very difficult to consistently deliver the active form of GLP-1 because of its short half-life. Even when using the long acting form of GLP-1, exendin-4, twice daily administration is required to maintain a normal glucose level. Szayna M, Doyle M E, Betkey J A, Holloway H W, Spencer R G, Greig N H, Egan J M: Exendin-4 decelerates food intake, weight gain, and fat deposition in Zucker rats. Endocrinology 141:1936-1941, 2000
Gene therapy is generally considered as a promising approach, not only for the treatment of diseases with genetic defects, but also in the development of strategies for treatment and prevention of chronic diseases such as cancer, cardiovascular disease and diabetes. However, nucleic acids, as well as other polyanionic substances are rapidly degraded by nucleases and exhibit poor cellular uptake when delivered in aqueous solutions. Since early efforts to identify methods for delivery of nucleic acids in tissue culture cells in the mid 1950's, steady progress has been made towards improving delivery of functional DNA, RNA, and antisense oligonucleotides in vitro and in vivo.
The gene carriers used so far include viral systems (retroviruses, adenoviruses, adeno-associated viruses, or herpes simplex viruses) or nonviral systems (liposomes, polymers, peptides, calcium phosphate precipitation and electroporation). Viral vectors have been shown to have high transfection efficiency when compared to non-viral vectors, but due to several drawbacks, such as targeting only dividing cells, random DNA insertion, their low capacity for carrying large sized therapeutic genes, risk of replication, and possible host immune reaction, their use in vivo is severely limited.
An ideal transfection reagent should exhibit a high level of transfection activity without the need for any mechanical or physical manipulation of cells or tissues. The reagent should be non-toxic, or minimally toxic, at the effective dose. It should also be biodegradable in order to avoid any long term adverse side effects on the treated cells. When gene carriers are used for delivery of nucleic acids in vivo, it is essential that the gene carriers themselves be nontoxic and that they degrade into non-toxic products. To minimize the toxicity of the intact gene carrier and its degradation products, the design of gene carriers needs to be based on naturally occurring metabolites.
As compared to viral gene carriers, there are several advantages to the use of non-viral based gene therapies, including their relative safety and low cost of manufacture. There are several polymeric materials currently being investigated for use as gene carriers, of which poly-L-lysine (PLL) is the most popular, but few of them are biodegradable. In general, polycationic polymers are known to be toxic and the PLL backbone is barely degraded under physiological conditions. It remains in cells and tissues and causes an undesirably high toxicity. In addition, like most cationic polymers, PLL/DNA complexes have drawbacks including precipitation as insoluble particles and the tendency to aggregate into larger complexes under physiological conditions.
In view of the foregoing, there is a need for the development of a composition and a gene therapy method for the treatment of type-2 diabetes wherein the gene carrier is soluble and biodegradable, meaning that the non-viral polymer gene carrier can break down or degrade within the body to non-toxic components after the genes have been delivered.